mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Sea Slug Acquires Photosynthetic Gene Through Horizontal Gene Transfer
The Sea Slug Elysia Chlorotica is a very unique creature that utilizes photosynthetic properties coming from the prey it consumes. Photosynthetic plastids are ingested when the slug consumes its primary prey Vaucheria litorea, and are sequestered by the mollusc's digestive epithelium. These plastids continue to photosynthesize for months despite the absence of algal nucleocytoplasm. This has puzzled researchers because plastid metabolism relies heavily (>90%) on the nuclear genome for the proteins necessary in these processes. Two prevailing theories emerged in the face of this phenomenon: either the plastids are able to retain genetic autonomy or the mollusc is somehow providing necessary photosynthetic helper proteins. The group goes on to show that the lateral theory is accurate, and that the nuclear gene oxygenic photosynthesis (psbO) is expressed in the sea slug. In fact, the gene has integrated into the germline. Plastid Acquisition and Function Sea slugs that have obtained plastids from consuming litorea do not pass these on vertically. Each generation of sea slugs must consume fresh litorea in order to gain the psbO gene which enables them to produce the exogenous photosynthetic proteins. Cholortilca is unique as it provides and opportunity to study horizontal gene transfer (HGT) between species. Specifically, it would advance knowledge regarding transfer between two multicellular eukaryotes. It is not often you see a molecular symbiotic relationship between and alga and a mollusc. Once the plastids have been consumed they stop dividing and line the the digestive diverticula. The plastid continue photosynthesis, however, which generates valuable resources to the sea slug like energy and carbon. The plastids continue their work throughout the slugs ten month life span, in spite of being entirely devoid of an algal nucleus to provide proper enzymes for photosynthesis and housekeeping. HGT of psbO Plastid transfer is known to occur when two organisms come into extremely close proximity. During feeding the sea slug epithelium contacts the nuclei of the litorea. When the plastids and other cellular matter are ruptured in the gut chromosomal DNA could possibly transferred in mass, There is an opposing belief that the psbO gene is being transmitted virally. Regardless of the mechanism of HGT it has become clear that the psbO gene has been integrated into the germline of the Cholortilca. ''While each generation of slugs has to consume ''litoria to gain the plastids that enable them to photosynthesize and produce energy, the psbO gene products are already present (Figure 3). psbO is a nuclear gene that encodes a manganese stabilizing protein that operates in the photosystem II complex. It plays and essential role in converting light energy to chemical energy that the organism can utilize. The group showed that the psbO gene had gone germline in the sea slug by looking at newly fertilized eggs, as older sea slugs could have residual psbO from litorea that had already been ingested. As they had originally thought the eggs also contained the psbO gene showing that the sea slug had incorporated the gene into its germline, which allows for a symbiotic (parasitic?) relationship with the algae. References 1. Mary E. Rumpho, Jared M. Worful, Jungho Lee, Krishna Kannan, Mary S. Tyler,Debashish Bhattacharya, Ahmed Moustafa, and James R. Manhart. Horizontal gene transfer of the algal nuclear gene psbO to the photosynthetic sea slug Elysia chlorotica. PNAS 2008 105 (46) 17867-17871; published ahead of print November 11, 2008,doi:10.1073/pnas.0804968105